Photometers and spectrophotometer can perform a variety of functions, such as measuring spectral properties of discrete solid, liquid or gas samples, and providing in-situ or on-line continuous measurement and control of manufacturing processes. Photometers generally measure changes in light intensity over time, while spectrophotometers generally measure light levels over a spectrum of colors or wavelengths. These instruments are therefore useful in many applications, including the monitoring and control of industrial processes such as chemical analyses and biological fluid analyses, and in the manufacturing of optical coatings.
Quality photometers require stable and low noise baseline signals over time while quality spectrophotometers require stable, low noise, and linear baseline signals over a spectrum of colors, as well as during the time measurements are being taken. Therefore, conventional photometers and spectrophotometers generally require high-performance components such as very stable and precise power supplies, light sources, detectors, mirrors, moving parts, and electronics. Furthermore, such instruments must be designed to compensate for any residual non-linearities and noise.
Another important requirement is the need to the control or compensate for stray, scattered, or extraneous light that may occur or be present during the measurements. Therefore, conventional instruments must usually be kept in tightly-controlled environments in order to obtain a high degree of stability for extended periods of time. This is required to minimize the noise that mixes with the signal, for example, low frequency noise such as signal drift. Although conventional instruments incorporate high-performance components to minimize such noise, the need for initial, periodic, and even continuous baseline correction remains. Oftentimes, a beam-splitter will be inserted before the sample to create a less than ideal quasi-reference beam.
Process photometers often require a light chopper or a pulsed light source combined with a lock-in amplifier to help provide precise measurements. However, chopper/lock-in add-ons do not eliminate all sources of noise. In fact, light choppers and lock-in amplifiers often add other noises to the signal, while increasing cost and complexity.
Monitoring systems, for example, are often used in the manufacture of coatings. Optical coatings modify the reflectance, absorbance, and transmittance properties of optical components. Due to the interactions of light with extremely thin films of optical materials, it is essential that the thicknesses of the coatings be accurately controlled so that the optical components achieve the desired properties required for the corresponding applications. Conventional monitoring systems measure the growth of thin-film optical coatings by providing in-situ transmittance and/or reflectance measurements of a stationary witness sample while it is being coated along with the nearby moving production substrates. The production parts are usually placed on rotating plates, domes, or planetary fixtures in order to make the coatings substantially uniform in thickness regardless of their location in the supporting structure.
However, the approaches offered by conventional monitoring systems often suffer from several limitations, including the generation of a signal which is contaminated by various sources of noise and the lack of stability of the witness glass. Such noise may originate from process heating and cooling components, mechanical flexing of the chamber, vibrations from on-board equipment such as motors or pumps, emission changes in the light source and noise from the detectors or electronics.
Additional signal problems arise from the coating, creep or deterioration of other optical components in the light path, such as the optical port windows of the coating chamber. Still more noise can originate from intense and highly variable nearby background light sources such as process heaters, thermal sources, and electron beam guns. Conventional optical monitors use a variety of schemes with limited success to combat these problems. These schemes include but are not limited to: use of choppers and lock-in amplifiers to modulate and demodulate or extract most of the signal from the noise, use of beam splitters and reference detectors to monitor the light beam just before it enters the process tank, placement of the monitoring samples and other optics such as to minimize impact from the above process effects, the use of optical fibers, the use of electronic filters to remove as much noise as possible, and digitization of the signals as soon as possible after it emerges from the process vessel.